1. Field of the Invention
The present invention relates to an image data conversion apparatus for converting input image data for four colors consisting of cyan, magenta, yellow and black into output image data, an image data conversion program storage medium storing an image data conversion program which causes a computer to operate as such an image data conversion apparatus, and an image data re-conversion program storage medium.
2. Description of the Related Art
Hitherto, to perform a printing using a printing machine, in view of the fact that considerable work and cost are needed for operating the printing machine, there is adopted, prior to the printing, such a procedure that a color printer or the like is used to create a proof image in such a manner that the same finish of an image as much as possible can be obtained as compared with the finish of an image printed by the printing machine, and the printing is performed after an acceptable result is obtained on the proof image.
When it is intended to produce a printed matter wherein for example a secret portion of a document is masked with a black, or a portion of one's eyes is masked for the purpose of protection of one's privacy, an image of the portion to be masked is erased in such a manner that with respect to the portion on an image, of four colors of cyan (c), magenta (M), yellow (Y) and black (K), which are colors of ink to be used for printing, dot % for K is established with 100%, and dot % for C, M and Y is established with 0%. Thus, it is possible to obtain a printed matter in which the intended portion is masked. While the establishment of dot % for K with 100% makes it possible to implement the maximum density of the possible image density range on the printed matter, dot % for C, M and Y is established with 0% nevertheless. The reason why this is to do so is that in the event that an image of the portion to be masked is expressed by any of C, M and Y, when the masked portion is observed through reflection of light on the printed matter, there is a possibility that the masked portion appears in some extent in some reflection conditions of light.
To produce a proof image of an image including such a mask as mentioned above, let us consider a color printer for producing a proof image in such a manner that light sources for three colors of red (R), green (G) and blue (B) are used to expose a printing paper or the like to light.
In this case, image data consisting of four colors of C, M, Y and K is converted into image data consisting of three color of R, G and B, so that the color printer produces the proof image in accordance with the image data consisting of three color of R, G and B. A point or an area, in which the dot % of K on the image is 100%, offers the maximum density. Accordingly, even if any value is established for dot % of C, M and Y, it is converted into R=G=B=0 with which a black involved in the maximum density appears on the printing paper. Thus, even if images for C, M and Y are erased with respect to the masked portion, or alternatively even if an image for any one of C, M and Y appears on the masked portion owing to forgetting about erasing the images, the completely same proof image will be produced in both the cases.
For this reason, even if there is performed a printing upon confirmation of various check points on the proof image, forgetting about erasing the images for C, M and Y of the masked portion would make such a possibility that there is produced a printed matter in which not only ink for K 100% but also inks for C, M and Y are applied to the masked portion, so that a rough state of the image of the masked portion appears in some reflection conditions of light.
In this respect, there will be described more in detail in conjunction with drawings.
FIG. 7 is a view showing a color image of an original before masked. FIG. 8 is a view showing a monochromatic image wherein the color image of FIG. 7 is subjected to a color separation into monochromes of C, M, Y and K. FIG. 9 is a view showing a color image, wherein a portion of eyes of the color image of FIG. 7 is masked, which is intended to be finally obtained as a printed matter. Each of FIGS. 10 and 11 is a view showing a monochromatic image wherein the color image of FIG. 9 is subjected to a color separation into monochromes of C, M, Y and K. FIG. 10 shows separation images in which images of portions corresponding to the mask, of C, M and Y are not erased. FIG. 11 shows separation images in which images of portions corresponding to the mask, of C, M and Y are erased.
In the event that a color printer is used to produce a proof image, even if image data, in which images of the portions of the mask of C, M and Y are not erased, as shown in FIG. 10, is converted into image data for a color printer, it is possible to obtain the same image data as a case where image data, in which images of the portions of the mask of C, M and Y are erased, as shown in FIG. 11, is converted into image data for a color printer. Thus, when a proof image is outputted in accordance with image data thus obtained, it is possible to obtain a proof image, as shown in FIG. 9, in which images of C, M and Y of the portions of the mask do not appear at all.
FIG. 12 is a view showing a color image, which is printed in accordance with the image data corresponding to the separation image of FIG. 10. FIG. 13 is a view showing a color image, which is printed in accordance with the image data corresponding to the separation image of FIG. 11.
In the event that a printing is performed in accordance with image data, in which images of C, M and Y of the portions of the mask are inadvertently not erased, as shown in FIG. 10, it happens that images of C, M and Y of the portions of the mask appear in some reflection conditions of light on a printed matter, as shown in FIG. 12. Consequently, when a printing is performed, there is a need to perform the printing in accordance with the image data, in which images of C, M and Y of the portions of the mask are erased, as shown in FIG. 11, and thereby obtaining a printed matter completely free from vestiges of images of the portions of the mask, as shown in FIG. 13.
As it is, it cannot be recognized at all on the proof image that it is forgotten to erase images of C, M and Y of the portions of the mask. Thus, there is a possibility that a printed matter, which can be discriminated in the image of the portion of the mask, as shown in FIG. 12, is inadvertently produced in accordance with the image data, in which images of C, M and Y of the portions of the mask are inadvertently not erased, as shown in FIG. 10.
In this case, there is a need to do over again in producing a form plate in which images of C, M and Y of the portions of the mask are erased, and also to do over again in printing. This involves very large waste of labor and cost.
Further, in the event that such a defective printed mater is forwarded, there is a possibility that it involves a further large problem.